The present invention relates to implantable medical devices, and more particularly to a voltage converter, for use within an implantable Spinal Cord Stimulation (SCS) system, or similar implantable device, that uses a switching regulator to provide a voltage step-up function.
Many implantable medical devices, such as neural stimulators, sensors, and the like, utilize a battery as a primary source of operating power. Other types of implantable devices, such as known cochlear stimulators, rely on transcutaneous inductive power transmission from an external device to the implantable device, where an induced voltage is thereafter rectified and filtered in order to provide the primary operating power for the implantable device. In both types of devices, a battery-powered device or an RF-powered device, there: is a frequent need to derive other operating voltages within the device from the primary power source. That is, there is a frequent need to step up the voltage of the primary power source to a higher voltage in order to, e.g., generate a high stimulation current.
In order to perform the voltage step-up or step-down function, it is known in the art to use a charge-pump voltage converter circuit. Charge pump circuits typically rely on a network of capacitors and switches in order to step up and step down a primary power source. For example, in order to step up a primary voltage source, a network of capacitors, e.g., four capacitors, may be connected in parallel through a switching network, and kept in the parallel connection until each capacitor is charged to the voltage level of the primary power source. In systems where a battery is used as the primary power source, the battery voltage is the voltage of the primary power source. Once thus charged, the capacitors are switched so that they are connected in series, thereby effectively creating a voltage across the series connection that is an integer multiple of (in this example, four times) the voltage of the primary power source. The charge associated with this higher voltage may then be transferred to another capacitor, e.g., a holding capacitor, and this process (of charging parallel-connected capacitors, switching them in series, and then transferring the charge from the series connection to a holding capacitor) is repeated as many times as is necessary in order to pump up the charge on the holding capacitor to a target voltage that is higher than the voltage of the primary power source.
While charge-pump circuits have proven effective for performing step up and step down functions, such circuits require a large number of capacitors to provide fine resolution of voltage. Charge pump circuits with course voltage resolution (i.e., a small number of capacitors) result in inefficient power use because the current resulting from the excess voltage is dissipated. Charge pump circuits with fine resolution require large numbers of capacitors, and are therefore not well suited for small implantable medical devices. Moreover, charge pump circuits tend to be relatively slow (i.e., are limited in peak current) and inefficient in operation.
Switching regulators have been used to overcome some of the limitations of charge-pump circuits. Known switching regulators utilize a closed loop voltage control. Such closed loop voltage control includes a resistor divider to sample either the voltage output of the switching regulator, or the voltage of an energy storage device being charged by the switching regulator, e.g., a holding capacitor. The voltage of the node between the resistors is compared to a reference voltage and the difference between the reference voltage and the node voltage is used by control logic to turn the switching regulator on or off, and thereby control the output voltage. Unfortunately, the use of resistors in such a circuit consumes energy continuously. Further, known switching regulators use a single duty cycle, and the duty cycle of a switching regulator that is best for low voltages may not be the best duty cycle for high voltages.
What is needed, therefore, is a voltage converter circuit that is able to perform the step up function, efficiently, quickly, and without having to rely on the use of a large number of capacitors, and which operates more efficiently than known switching regulators.